The present invention pertains to a device and a process for extruding homogeneous plastic articles, particularly pipes and will be described in connection therewith.
Plastic pipes are currently used in mains because of their light weight, their ease of installation and connection and their corrosion resistance.
These plastic pipes, which have been known for several decades, are always manufactured according to the original principle using a device as shown in FIG. 1.
This device of extrusion head, fed with molten plastic by an extruder to which it is attached in the axis of extrusion, is formed by two principal parts between which circulates the molten material up to the outlet of the extrusion die. The central part forming the internal blank of the pipe comprising the punch and mandrel is schematically formed by two conical parts joined to each other in their largest base, the punch being the diverging cone relative to the outlet of the extruder and the mandrel being the part converging toward the outlet of the die. This mandrel is attached by means of fins to the part forming the external part of the device so as to leave a free tubular conduit between the two parts of the outlet of the extruder at the outlet of the die of the said device.
This device has changed little over time, whereas the performance of the extruder to which it is attached has progressively increased and they have ever increasing material throughputs. Despite the care taken by the manufacturers of these machines, this leads to notable temperature differences in the stream of molten plastic discharged by these extrusion machines, causing considerable differences in the viscosity of the plastic between the different points of the device. These variations in viscosity generate fluctuations in throughput and lead to appreciable deviations in thickness over the finished article such as the pipe. To remedy this disadvantage, the machine designers have been steadily increasing the volumes of material contained in this device to reduce these deviations in temperatures or even to correct the thickness deviations at the outlet of the device by various automatic centering systems, either by mechanical displacement of the punch and mandrel relative to the die or by creating different temperatures at the periphery of the die. Such correction systems require elaborate means for continuous measurement of the thickness over the circumference of the finished pipe.